Revert "[lit] Keep original cfg file case around."
This reverts commit bc3f54de1827e58655c34477d09211cbc42589bd.
The patch breaks in the following two scenarios:
- When manually passing an absolute path to llvm-lit with a lower-case drive letter: python bin\llvm-lit.py -sv c:\llvm-project\clang\test\PCH
- When the PWD has a lower-case drive letter, like after running cd c:\ with a lower-case "c:" (cmd's default is upper-case, but it takes case-ness from what's passed to cd apparently).